How well do first flowering dates measure plant responses to climate change? The effects of population size and sampling frequency

1. First flowering dates are occurring earlier than they did in the past in many locations around the world. It is sometimes assumed, implicitly or explicitly, that the changes in first flowering dates describe the phenological behaviour of entire populations. However, first flowering dates represent one extreme of the flowering distribution and may be susceptible to undesirable confounding effects. 2. We used observations of flowering in Colorado and Massachusetts to test whether changes in population size and sampling frequency affect observations of first flowering dates. 3. We found that the effect of population size on first flowering dates depended on location. Changes in population size were strongly related to the dates on which first flowering was observed in Massachusetts but not in Colorado. The lack of a significant effect in Colorado may reflect the rapid onset of spring after snowmelt and fixed developmental schedules of the plants at this sub-alpine site, or the scale of the plots sampled during the study. 4. We also found that changes in sampling frequency can influence observed changes in first flowering dates and other aspects of the flowering distribution. Similar to the effect of declines in population size, lower sampling frequency caused later observations of first flowering. However, lower sampling frequency, if maintained consistently throughout a study, did not significantly affect estimates of changes in flowering dates over time or in response to climate. 5. Synthesis. Researchers should consider the effects of changes in population size and sampling frequency when interpreting changes in first flowering dates. In some cases, past results may need to be reinterpreted. When possible, researchers should observe the entire flowering distribution or consider tracking peak or mean flowering dates to avoid the confounding effects of population size and sampling frequency.